I started at a week old, I gave them grit, boiled eggs (their favorite), bugs, worms, and oatmeal! I have to warn you, they will get spoiled quickly and not want to come see you for you.. just the food! I had to cut back so they would love me again! LOL!!
Now, IMO. You might do a search and find the BYC treats chart. Cooked egg or oatmeal is a common early treat, or yogurt. Keep in mind they can't digest lactose so you want to keep the quantity of yogurt pretty limited, but a little gives them some probiotic. And in case you haven't already read it about a dozen times, they will need a source of grit for many foods. For chicks I use the fine stuff at the bottom of my grit bag for the hens. Sand also works for chicks.
My 6 week olds just got their first treat other than mealworms today. They got some leftover banana and lettuce. Didn't care too much about the lettuce (as they are just obsessed with stealing blades of grass from each other at the moment) but they loved the banana and were all trying to drag it away.